Population Of Rotational Levels Research Articles

State resolved detection of Cl, Br, CIO and BrO

Nascent CIO and BrO products in photodissociation or reaction processes were observed in a state resolved manner by applying the two-photon laser-induced fluorescence technique, where two simultaneously absorbed laser photons excite the products to the C-state. CI and Br atoms can be observed by means of three-photon excitation with subsequent recording of the VUV fluorescence light. The advantage of this detection technique, other than the simpler laser system, is the use of low energy photons which don't interfere with other processes. State resolved detection of BrO products has been performed in the reactions 0(1D)+CF3Br and Br+03.The photodissociation of OCl0(2A2v1,v2,v3)→Cl0(X2πω, v,J,λ)+0(3P) in the 356–371 nm region is clearly a function of the initial type of motion of OC10(2A2vl,v2,v3). The population of the fragment rotational levels is observed to resemble Boltzmann distributions of around 1000 K. When high vibrationally excited states of the symmetric stretch mode of OC10(2A2v1>17,v2,v3) are excited, then most of the available energy is transferred to CIO vibration. Those extremely vibrationally excited CIO products carry sufficient energy to react with ambient nitrogen in the atmosphere, which is confirmed in a final end product analysis where the formation of N20 is observed.The Cl quantum yield of the minor OCIO(2A2 v1,v2,v3) → CI(2PJ) + 02 channel depends on the state of the parent molecule and is of the order of 4%.

The electron beam technique in carbon monoxide

Abstract The electron beam fluorescence technique was developed to measure the rotational level population in the ground state of a carbon monoxide molecule. The data on rotational transition probabilities under ionization–excitation of CO (X 1 Σ + ) into the CO + (B 2 Σ + ) state by high energy electrons are presented, which noticeably differ from those predicted by the dipole excitation model. A flow in a free jet was used to obtain the experimental data in a low temperature rotationally equilibrium gas target. The probability matrix is represented in a compact form corresponding to the sudden approximation model.

Formation of OH(v=0,1) by the Reaction of Fast H with O3

A two-laser, pump−probe experiment has been used to determine the rotational level population distribution of OH(v=0,1) resulting from the reaction of fast (2.3 eV) hydrogen atoms with ozone. A trace amount of H2S in slowly flowing O3 was photolyzed at 193 nm, and the resultant OH was detected using laser-induced fluorescence (LIF). The pump−probe delay time was adjusted in order to verify negligible relaxation of the nascent OH product. Initially, side reactions such as O(1D) + H2S → OH + HS were found to contribute to the OH signals; they were subsequently eliminated by adjusting the reactant concentrations and flow velocity. The resultant OH LIF spectra were corrected for several factors using either known or measured experimental quantities, including OH(A) collisional quenching, baseline drift, and partial saturation of the OH(A−X) absorption lines. Near-gas kinetic rate constants for OH(A,v‘=0,N‘≤25) collisional quenching by O3 were derived. The corrected spectra were fit using a nonlinear least-squ...

State-to-state rate coefficients for rotational relaxation of CO in Ar

The rotational level populations of CO molecules were measured in CO (<10%)+Ar free jets by electron beam fluorescence (in a stationary jet) and resonantly enhanced multiphoton ionization (in a pulsed jet). The measured evolution of the nonequilibrium rotational energy was used to derive the rotational relaxation cross sections in the temperature range from 7 to 150 K. Using the pressure-broadened linewidth data the state-to-state rotational relaxation rate coefficients were found using the most popular fitting law forms: The modified exponential gap, the statistical polynomial-exponential gap, and the energy corrected sudden approximation. The fitted rates were compared with the experimental and theoretical data presently available in the literature. They were checked also by consideration of the rotational kinetics in free jets and by comparison between the computed and experimental rotational level populations both in near and far regions of a flow.

Microwave electronic spectrum of the Ne···Ne+ long-range complex

We have used an ion beam method to observe 276 transitions in the microwave electronic spectrum of the long-range Ne···Ne+ ionic complex, involving levels which lie within 10.2 cm-1 of the lowest dissociation limit. An electric field dissociation technique has been used to access directly levels with binding energies of up to 6.8 cm-1 and we have been able to construct an experimental energy level pattern for at least 17 vibration–rotation progressions in this region of the potential. Microwave–microwave double resonance and Zeeman effect measurements have been crucial in establishing this energy level pattern. The levels are populated by the vertical electron impact ionisation of the neutral neon dimer, produced by means of a nozzle beam, with some population of higher rotational levels of the dimer ion probably resulting from larger cluster fragmentation. A case (c) effective Hamiltonian analysis has been successful in describing the levels in six of the progressions but a full theoretical analysis requires a coupled-states calculation. Analysis of Zeeman splittings for the microwave lines identifies the J values of the levels involved. The observed g-factors provide information about the transition from case (c) to case (e) coupling as both the rotational quantum number increases and the energy levels become more weakly bound.

Populations of rotational levels of nitrogen molecules in free jets. Comparison of CARS and electron beam fluorescent technique

The paper presents experimental data on the distribution of the rotational level populations of nitrogen molecule ground state, N 2( X 1Σ + g , v=0). The results have been obtained on the axis of the free jet in stationary and impulse regimes. Two diagnostic techniques (the CARS and Electron-Beam Fluorescent) were used. The perfect coincidence of the results obtained by these two methods in the identical conditions is shown. Good agreement has been also obtained with the results of measurements by LIF and Raman spectroscopy methods. All methods reveal a breakdown of Boltzmann distribution of rotational level population time during gas jet expansion into vacuum. It is found that additional disturbance of Boltzmann distribution is connected with the release of the energy of condensation into gas jet under high stagnation pressures.

Fine‐Structure analysis of ClO fragments in the mode‐specific predissociation of chlorine dioxide

AbstractThe photodissociation of OClO (2A2) ClO (X2πΩ,ν,J,Λ)+O (3P) in the 424‐450 nm region and in the 356‐371 nm region has been investigated by monitoring the ClO (X2πΩ,ν = 0,J,ν) fragments using two‐photon laser‐induced fluorescence. In the long‐wavelength regime the product vibrational excitation is clearly a function of the particular initial type of motion of OClO (2A2v1,v2,V3). This behavior is significantly reduced at shorter photolysis wavelengths. The population of the fragments rotational levels is observed to resemble Boltzmann distributions of around 1000 K. A population ratio of about P(2π3/2): P(2π1/2) = 3.5 is obtained for the two spin‐orbit states of ClO. The ν‐doublet states (A′) and (A″) of the ClO fragments are equally preferred in the dissociation process.

Picosecond laser-induced fluorescence measurement of rotational energy transfer of OH A^2Σ^+ (v′ = 2) in atmospheric pressure flames

Rate constants for total and state-specific rotational energy transfer (RET) of OH A(2) ?(+) (v? = 2) have been measured directly in atmospheric methane-air and methane-oxygen flames for the first time to our knowledge. We used a picosecond Raman-excimer laser (tau(l) = 300 ps, lambda = 268 nm) to excite the P(11) (12.5) and Q(11) (16.5) A -X transitions in the (2, 0) band of OH molecules. We analyzed the resultant fluorescence with a high-resolution spectrometer in combination with a fast-gated, intensified CCD camera (tau(g) = 400 ps). We recorded the temporal evolution of the emission spectrum by shifting the detection time with respect to the laser pulse. Measured emission spectra were inverted to yield the time-dependent population of rotational levels in the excited state. We calculated rate constants for RET from the results of the fit. The total RET in v? = 2 is similar to v? = 0, 1. The state-specific rates are represented well by a simple energy-gap law.

Molecular Hydrogen Absorption in the [CLC][ITAL]z[/ITAL][/CLC] = 1.97 Damped L[CLC]y[/CLC]α Absorption System toward Quasi-Stellar Object Q0013−004

We present a new ultra-violet spectrum of the QSO 0013-004 with 0.9 \AA resolution obtained with the MMT Blue spectrograph. The \upsilon = 0 - 0, 1 - 0, 2 - 0 and 3 - 0 Lyman bands of H_2 associated with the z = 1.9731 damped Ly alpah absorption line system have been detected. The H_2 column density is N(H_2) = 6.9 (\pm 1.6)\times 10^{19} cm^{-2}, and the Doppler parameter b = 15\pm 2 km/s. The populations of different rotational levels are measured and used to derive the excitation temperatures. The estimated kinetic temperature T_K\sim 70 K, and the total particle number density n(H) \sim 300 cm^{-3}. The UV photoabsorption rate $\beta_0 \sim 6.7\times 10^{-9}$ s^{-1}, about a factor of few times greater than that in a typical diffuse Milky Way interstellar cloud. The total hydrogen column density is $N(H) = 6.4(\pm 0.5)\times 10^{20} cm^{-2}$. The fractional H_2 abundance f = 2N(H_2)/(2N(H_2) + N(H I)) \sim 0.22 \pm 0.05 is the highest among all observed damped Ly\al absorbers. The high fractional H_2 abundance is consistent with the inferred presence of dust and strong C I absorption in this absorber.

Rotational relaxation of nitrogen in helium

Rotational level populations of N 2 have been measured in free jets of He + N 2 mixtures by an electron beam technique. The experimental data are compared with kinetic calculations, based on theoretical state-to-state rate constants, obtained by the IOS method with an M3SV potential energy surface. Reasonable agreement of the data substantiates the IOS method down to low (≈ 10 K) temperatures. Other available calculations of rotational relaxation times are compared with the present experimental data.

Production of vibrationally and rotationally excited NO in the night time terrestrial thermosphere

A quantitative interpretation is given of the observed quiescent nighttime radiance of nitric oxide in the fundamental vibration‐rotation band near 5.3 μm. The radiance measured in the space shuttle experiment Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS‐1A) is known to have two components, one characterized by a thermal population of rotational levels and the other by a highly excited rotational population. The analysis presented here confirms that the thermal population is due to impact excitation of NO by atomic oxygen and attributes the highly excited distribution to the reaction of N(4S) atoms with O2. The measured nighttime emission profile is compared with predictions for several model atmospheres. Both sources of excited NO depend upon the latitude, longitude, local time, and geomagnetic indices. The fraction of vibrationally excited NO produced by the reaction of N(4S) with O2 increases rapidly with altitude from 130 to 200 km and its contribution to cooling, though much less than that from inelastic excitation of NO(v=0) is, at higher altitudes, comparable to cooling produced by the atomic oxygen fine‐structure line at 63 μm.

N,K Rotational Level Populations of CD3 from the 266 nm Photofragmentation of |JKM) State-Selected CD3I

N,K Rotational Level Populations of CD3 from the 266 nm Photofragmentation of |JKM) State-Selected CD3I

Probabilities of rotational transitions under ionization into N2+ (B 2Σu+, v' = 0) state by electron impact

The results of measurement of the rotational transition probability matrix for ionization-excitation of N2(X 1Σg+) into the N2+ (B 2Σg+) state by a high energy electron beam are presented. Experimental data are obtained in pure nitrogen and argon-nitrogen free jets at low temperatures under equilibrium conditions. The matrix is represented in a compact form corresponding to the infinite order sudden approximation with the transition probabilities from the ground rotational state of N2(X 1Σg+, N = 0) into the excited N2+ (B 2Σg+, N− = 1, 3, 5, 7) state equal to P10 = 0.8251, P30 = 0.1494, P50 = 0.0196, P70 = 0.0045. The results obtained are compared with the results of measuring populations of rotational nitrogen levels by LIF, CARS and Raman spectroscopy methods. An excellent agreement has been found which confirms the validity of obtained probabilities.

Diode Laser Study of the Rotational Redistribution in CH3OH during a Supersonic Jet Expansion

After rapid cooling in a supersonic jet expansion, the populations of the lowest rotational levels of methanol have been determined from the intensities of IR absorption transitions of the C-O stretching band at 10 μm. A newly constructed multireflection system (White type, 16 passes) has been employed for the detection of weak absorption lines. The rotational temperature of methanol has been found to be lower for the E-symmetry substate of the ground (vt = 0) torsional state than for the A-symmetry substate: Trot(A-Sym) = (15.8 ± 0.5) K and Trot(E-Sym) = ( 13.3 ± 0.9) K. No torsional hot-band transitions have been observed in the jet spectrum, indicating an efficient cooling of the torsional degree of freedom. The population distribution of the probed rotational levels is strongly nonthermal, or "normal," in the sense that the A/E population ratio maintains its room temperature value of near unity.

The monoenergetic unimolecular reaction of expansion-cooled NO2: NO product state distributions at excess energies 0–3000 cm−1

We report detailed vibrational, rotational, and electronic (V,R,E) distributions of nascent NO(X 2Π1/2,3/2) deriving from monoenergetic unimolecular reactions of expansion-cooled NO2. Near UV excitation above dissociation threshold (25 130.6 cm−1) prepares molecular eigenstates which are admixtures of the optically active 1 2B2 state and the ground X̃ 2A1 electronic state. The strong mixings among the vibronic states result in vibrational predissociation from states of predominantly ground state character, and the NO product state distributions (PSDs) are compared with the predictions of several statistical theories. The PSDs are combined with previously measured O(3PJ) distributions and unimolecular reaction rates, thereby providing a complete description of the decomposition of NO2 at these excess energies. All the rotational distributions show prominent fluctuations and structures, but tend on average to follow the statistical distributions predicted by phase space theory (PST). This behavior is observed in both NO(v=0) and NO(v=1) channels, although the relative population in NO(v=1) was always greater than expected by PST. The NO(v=1) fractional population is bounded by the predictions of the separate statistical ensembes (SSE) method, and recent variational Rice–Ramsperger–Kassel–Marcus (RRKM) calculations are in agreement with the experimental results. Prior distributions underestimate the degree of vibrational excitation even more than PST does, and also the relative populations of the lower NO rotational levels. The observed NO spin–orbit states are always colder than statistical. We conclude that a significant interplay between dynamical biases and statistical expectations is manifest from the onset of dissociation, and is particularly evident when the initial parent rotational state is well defined.

Energy transfer in the 31,214151 Fermi-resonant states of acetylene. I. Rotational energy transfer

An infrared–ultraviolet double resonance technique is used to probe the state-to-state rotational energy transfer dynamics of self-relaxation in acetylene. The output of an optical parametric oscillator at ∼3 μm is used to excite C2H2 to a rotational level within one of its Fermi-resonant 31,214151 states. By fixing this wavelength and scanning the frequency-doubled output of a tunable dye laser, laser induced fluorescence signals arising from collisional population of rotational levels within both dyads are observed and state-to-state rate constants for rotational relaxation are obtained. Rotational relaxation to J levels within the pumped (upper energy) Fermi-dyad accounts for 74% of the total rate of loss of the population of the J=12 level, whereas relaxation to J levels in its partner accounts for only 16%. A further 7% of the absolute rotational relaxation rate is accounted for by vibrational relaxation out of the mixed levels, leaving only 3%–4% of the total relaxation to be accounted for.

N2(a″ 1Σg+) metastable collisional destruction and rotational excitation transfer by N2

Quenching and rotational coupling rate coefficients have been measured for the J=4–10, v=0 levels of the a″ 1Σg+ metastable state of N2 in collisions with ground state N2. Laser absorption is used to monitor the population of rotational levels of the a″ 1Σg+ state following depletion of the population of one or more levels by optical pumping to other states. The observed time dependence of the recovery of population of the perturbed level and the collision induced growth and decay of the populations of adjacent levels are interpreted in terms of quenching to other electronic levels and excitation exchange among adjacent rotational levels. For the J=6, v=0 level of the a″ 1Σg+ state the rate coefficients extrapolated to zero discharge current at 300 K are 2.3±0.1×10−16 m3/s for electronic quenching and 1.1±0.6×10−16 m3/s for excitation transfer to the J=4 and J=8 levels in collisions with N2. Very similar rate coefficients were obtained for the J=4, 7, and 8 levels.

Computation of inelastic rotational rates and collision cross-sections for astrophysically important systems: H2+ H2O and the water maser

The water maser radiation from stellar regions which is observed on Earth arises from the stimulated emission of radiation from rotationally excited states of the water molecule. In order to model and understand the physical conditions pertaining in the regions from which the radiations originates it is necessary to know the rates with which the rotational energy level populations of the water molecules are changed. One of the main mechanisms, other than stimulated emission, through which such changes occur is by collisions of the water molecules with molecular hydrogen, which is the most abundant species in the regions of interest. The purpose of the present paper is to report preliminary results for the rotationally inelastic cross-sections and of rate constants for collisions between H2 and H2O molecules which are needed in the modelling of the water maser. The paper also summarises the ab initio computation of the interaction potential and its fitting to an appropriate analytic form suitable for use in the subsequent quantum-mechanical scattering calculations of the rotationally inelastic cross-sections.

The relationship between physical observables and the potential energy surface in the He-HF system

This paper examines the effect of infinitesimal functional variations in a rigid rotor He-HF potential surface on several different types of observables: inelastic cross sections, rate constants, and rotational energy level populations. The dynamics and kinetic observables studied were found to be sensitive to a large number of Legendre components of the potential with the region of highest sensitivity dependent upon the energy or temperature as well as the states related by the individual observable. Sensitivity to the entire surface tends to show a large degree of structure due to competition among sensitivities to the individual potential components. Significant information loss has been observed in the transition from microscopic to macroscopic observables.

The connection between experimental observables and the potential energy surface in the He-HT system

The effect of infinitesimal functional variations in a rigid rotor He-HT potential surface on several different types of observables - inelastic cross sections, rate constants, and rotational energy level populations - is examined. The dynamic and kinetic observables studied were found to be most sensitive to the V 1 ( r) component of the potential with the region of highest sensitivity dependent upon the energy or temperature as well as the states related by the individual observable. Sensitivity to the entire surface tends to show more structure due to interference among sensitivities to the individual potential components. While a reasonable amount of information on the underlying potential is retained, significant information loss has been observed in the transition from the microscopic observables to the macroscopic ones.